This invention relates to improved methods of making hollow, biaxially oriented, heat-set partially crystalline articles. In another aspect, it relates to biaxially oriented, heat-set hollow poly(ethylene terephthalate) containers having excellent mechanical properties and low permeabilities to carbon dioxide and oxygen gas. The containers according to the present invention exhibit excellent gas barrier properties, that is low gas permeabilities, and significant mechanical property improvement in hoop yield strength, thermal stability and creep-resistant over prior containers.
In order to improve several physical properties of hollow articles such as containers made from poly(ethylene terephthalate), it has been suggested that biaxially oriented poly(ethylene terephthalate) hollow articles, made by orientation blow molding from a preform or parison under conditions to provide biaxial orientation and concomitant crystallization, be further heat-treated at higher temperatures than the orientation blowing temperature to further increase the density by increasing the crystallinity of the hollow article. Such increasing of the density or crystallinity by heating after shaping under orientation conditions is commonly known as heat-setting.
Wyeth et al. in U.S. Pat. No. 3,733,309 suggests such a process. However, the heat-setting process is mentioned only in passing and no specific examples including heat-setting are present in the patent. Of course, the extra step would ordinarily add considerable expense to the bottle making process. Such heat-setting processes typically take many minutes per container and are thus not well-suited to commercial speed container manufacture.
Collins in U.S. Pat. No. 4,039,641 discloses heat-setting containers of an organic crystallizable synthetic thermoplastic polymeric material. Among such materials disclosed are high density polyethylene, polypropylene homopolymers and copolymers and polyesters such as poly(ethylene terephthalate) and poly(butylene terephthalate), including polyesters such as ethylene terephthalate/isophthalate copolymers. In a preferred embodiment, heat-setting is accomplished by blowing the plastic parison in a heated blow-mold, preheated to the heat-setting temperature.
It is stated in the Collins patent that the heat-setting temperature used is that normally encountered in heat-setting of oriented films or fibers made from the given plastic material. It is not stated, however, what heat-setting temperatures are "normal" for making oriented films or fibers from poly(ethylene terephthalate).
It is also disclosed in Collins that after heat-setting, the container should be cooled down to a temperature, for instance, below about 60.degree. C. In one example of Collins, the heat-setting temperature of the mold is 200.degree. C. and in the other, it is 140.degree. C.
In Japanese Patent Application No. 146,175, laid open Nov. 15, 1980, containers are stretch blow-molded under conditions to biaxially orient the polyester molecules. It is explained that as a result of the stretch blow-molding, the residual strain was large and that when heated subsequent to the molding, the residual strain was released, causing deformation of the container. To solve this problem, the reference recommends heat-setting the containers after blow-molding. It is also recommended that the heat-setting temperature in unstretched areas such as the neck be held to 95.degree.-125.degree. C. so that hazing will not occur in these areas. Other areas are heat-set at a higher temperature. It is recommended that the heat-setting of the highly strained areas of the container be in the range from 125.degree. C. to 235.degree. C.
In Japanese Patent Application No. 77,672, laid open June 21, 1979, is similar except that it is not taught to heat-set unoriented parts at a lower temperature than other parts. The highest temperature disclosed for heat-setting is 130.degree. C. and in the only specific example, the oriented blow-molded container is heat-set by contacting with the hot blow-mold kept at 130.degree. C. and then lowering the mold temperature to 100.degree. C. to prevent container deformation when the container is discharged from the mold. In this reference,, it is stated that hazing occurs when higher heat-setting mold temperatures are used.
In Japanese Patent Application No. 21,463, laid open Feb. 17, 1979, a blown poly(ethylene terephthalate) container was heat-set by heating the container to 140.degree. C. while still within the blow-mold.
In Japanese Patent Application No. 78,267, laid open June 11, 1978, there is disclosed stretch blow-molding a thermoplastic resin, in the example specifically poly(ethylene terephthalate) to make a hollow article, and while the article is still in the mold to introduce hot gases for the purposes of heat-setting. In the example, the hot gas is at 180.degree. C. The example does not disclose cooling the heat-set article before removal from the mold, but the description of the drawing does describe this as an alternative treatment, using normal temperature compressed gas to cool the molded piece.
In Japanese Patent Application No. 66,968, laid open May 29, 1979, methods of reducing residual strain in biaxially oriented blown containers are disclosed. The methods are applied to unidentified, saturated polyester resins. In all of the methods, the container is heated after being formed by biaxial orientation blow-molding by one method or another. After the heat treatment the container is cooled, but the temperature to which the container is cooled is not disclosed. The heating step apparently includes heating the neck portion of the container, since in one method the heating is by passing steam through channels which include channels next to the neck, and in another method heating is carried out by high temperature pressurization of the interior of the container, which of course includes the neck.
In Japanese Patent Application No. 78,268, laid open June 11, 1978, a stretch blow-molded hollow body, including those made from poly(ethylene terephthalate) is heat-set by introducing hot gas under pressure into the interior of the container while in the mold. After the heat-setting, normal temperature gas can be optionally blown into the article to cool the article before removal from the mold, or the heat-set body can simply be exhausted to atmospheric. In an example, the heated gas for heat-setting is at 200.degree. C. In the specific example, no cooling before removal from the mold was disclosed. Again, the heating includes heating of the neck portion of the bottle.
In Japanese Patent Application No. 41,973, laid open Apr. 3, 1979, it is disclosed to heat-set stretch blow molded containers, including those made from poly(ethylene terephthalate) by heating the blown containers at a high temperature and then rapidly cooling them to room temperature. Heat treatment can be within the mold while under pressure and the heating can be by means of a hot mold. It is disclosed that the heat treatment should be such that the density of the container following the heat treatment is no greater than 1.40 gms./cc. In the example given, steam at 179.degree. C. is used for heating the mold in the heating step.
U.S. Pat. No. 2,823,421 to Scarlett discloses heat-setting of poly(ethylene terephthalate) films using heat-setting temperatures of 150.degree.-250.degree. C. after orientation stretching. This patent does not state, however, what "normal" polyester film heat-setting temperatures are. It does disclose that for a film streteched three times in each direction that a heat-setting temperature of 200.degree. C. is preferred by Scarlett.
German Patent No. 2,540,930 discloses heat-setting of hollow articles. The blank or parison is blow-molded at 70.degree.-140.degree. C. and then cooled in the mold to below 70.degree. C. Thereafter, the container can be reheated to the heat-setting temperature in the same mold or in a different mold. The heat-setting temperature is said to be 140.degree. C. or higher. In the disclosed process, the entire container including the neck is heated in the heat-setting step to the same temperature and the neck of the container crystallizes to an opaque state.
In the Brady et al. patent, U.S. Pat. No. 4,233,022, a container oriented by blow-molding of polyester at 75.degree.-100.degree. C. is heat-set. Heat-setting is accomplished in a hot mold at a suitable heat-setting temperature; examples of such temperatures are given as 150.degree. to 220.degree. C. The patent features controlling different zones of the container at different temperatures, so that all the sidewall of the container is at the maximum heat-setting temperatures being used, but the finish or neck, for instance, is actually cooled to prevent crystallization thereof. In this patent after the heat-setting step, it is stated that the container is cooled to a self-sustaining condition.
Accordingly, a need exists for a process for the production, at commercially acceptable speeds, of biaxially oriented, heat-set polyester, particularly poly(ethylene terephthalate), containers which containers exhibit low gas permeability, high mechanical strength including creep-resistance, thermal stability and high hoop yield strengths.